The present invention relates to an AD converting device which is employed for converting an audio signal, musical signal, image signal, or like analog signal into a digital signal which may be subjected to, for instance, an analysis in the frequency domain and/or time domain.
A quantization error is incidental to an AD conversion because an AD converter has a limited number of quantization steps. Many attempts have been made to average maldistributed quantization errors through application of a dither to an original analog signal by using a random noise, as set forth in, for example, B. A. Blesser, "Digitization of Audio: A Comprehensive Examination of Theory, Implementation, and Current Practice", Journal of Audio Enginnering Society, October 1978, Vol. 26, No. 10.
FIG. 1 shows a prior art example. An analog signal X for conversion into a digital signal is applied to an analog to an analog adder 12 from an input terminal 11. A random pulse generator 13 is provided, which is an M-sequence pulse generator comprising a shift register. Random pulses generated in digital form by the random pulse generator 13 are converted by a DA converter 14 into an analog random noise signal N, which is supplied to the analog adder 12 for addition to the analog signal X. The adder output (X+N) is provided to an AD converter 15, whereby it is converted, at intervals of a fixed period Ts, into a digital signal of a predetermined number of bits.
The converted output (X+N).sup.Q (where Q indicates that the output is a digital signal) from the AD converter 15 is applied, as required, to a digital subtractor 16, wherein a random noise value (N).sup.Q supplied from the random pulse generator 13 is digitally subtracted from the converted output (X+N).sup.Q, providing an output (X+N).sup.Q -(N).sup.Q at an output terminal 17.
In the conventional AD converting device of the type that superimposes the random noise on the analog signal to be converted, it is possible to distribute quantization errors uniformly over the quantization step size .DELTA. when the amplitude of the random noise N is selected to exceed about one-half the quantization step size .DELTA. of the AD converter 15. An increased amplitude of the random noise N will also lessen the influences of the nonlinearity of the quantization characteristic of the AD converter 15 and variations in its quantization step size owing to averaging effects. From this point of view, the random noise N may preferably be large in amplitude.
In the case where the random noise N has a small amplitude but the analog signal to be converted has a large amplitude, small noises will be masked by the signal if it is reproduced into an audio signal. Therefore, the digital subtractor 16 in FIG. 1 can be left out. Furthermore, when the output at the output terminal 17 is subjected to a digital fast Fourier transform (FFT) or digital discrete Fourier transform (DFT) for a frequency analysis, if the random noise N is small, the resultant signal component appearing on the observation screen may have high peaks above the low random noise levels distributed in the analyzing frequency range; and so the random noise is not obstructive to the observation of the signal component. Accordingly, the digital subtractor 16 can be dispensed with in this case, too.
On the other hand, when the amplitude of the random noise is increased as mentioned above, the noise component is distributed uniformly over a wide frequency band with high levels; therefore, it is necessary to remove the random noise component by means of the digital subtractor 16.
Moreover, according to the above prior art system, the random pulses from the random pulse generator 13 are converted by the DA converter 14 into an analog signal. In this case, if the amplitude of the random pulses (noise) is large, the nonlinearlity of the DA converter 14 cannot be ignored in the DA conversion and there arises the necessity of employing an expensive DA converter of a large dynamic range of conversion. Thus, the nonlinearity of the DA converter 14 prevents complete elimination of the random noise in spite of its subtraction by the digital subtractor 16.
Incidentally, there are shown in FIG. 2 variations in a quantization error produced when the amplitude of the random noise N was varied in the conventional AD converting device depicted in FIG. 1. The random noise amplitude on the horizontal axis in FIG. 2 is expressed in normalized form with the quantization step size .DELTA.. As will be seen from FIG. 2, an increase in the amplitude of the random noise N causes a decrease in the quantization error, but in this example, the error reaches a minimum value at a noise amplitude of 2.DELTA. and thereafter it is rather on the increase.